1. Field of the Invention
The present invention pertains to diode, thermionic, tunneling, and other devices that are designed to have very small spacing between electrodes and in some cases also require thermal isolation between electrodes. The invention may be applied to thermo-tunneling generators and heat pumps, and can be applied to similar systems using thermionic and thermoelectric methods. These thermo-tunneling generators and heat pumps convert thermal energy into electrical energy and can operate in reverse to provide refrigeration. The invention may also be applied to any device that requires close, parallel spacing of two electrodes with a voltage applied and current flowing between them.
2. Description of the Prior Art
The phenomenon of high-energy electron flow from one conductor (emitter) to another conductor (collector) has been used in many electronic devices and for a variety of purposes. For example, vacuum-tube diodes were implemented this way, and the physical phenomenon was called thermionic emission. Because of the limitations imposed by the relatively large physical spacing available, these diodes needed to operate at a very high temperature (greater than 1000 degrees Kelvin). The hot electrode needed to be very hot for the electrons to gain enough energy to travel the large distance to the collector and overcome the high quantum barrier. Nevertheless, the vacuum tube permitted electronic diodes and later amplifiers to be built. Over time, these devices were optimized, by using alkali metals, like cesium, or oxides to coat the electrodes, in an effort to reduce the operating temperature. Although the temperatures for thermionic generation are still much higher than room temperature, this method of power generation has utility for conversion of heat from combustion or from solar concentrators to electricity.
Later, it was discovered that if the emitter and the collector were very close to each other, on the order of atomic distances like 2 to 20 nanometers, then the electrons could flow at much lower temperatures, even at room temperature. At this small spacing, the electron clouds of the atoms of the two electrodes are so close that hot electrons actually flow from the emitter cloud to the collector cloud without physical conduction. This type of current flow when the electron clouds are intersecting, but the electrodes are not physically touching, is called tunneling. The scanning tunneling microscope, for example, uses a pointed, conducting stylus that is brought very close to a conducting surface, and the atomic contours of this surface can be mapped out by plotting the electrical current flow as the stylus is scanned across the surface. U.S. Pat. No. 4,343,993 (Binnig, et al.) teaches such a method applied to scanning tunneling microscopy.
It has been known in the industry that if such atomic separations could be maintained over a large area (one square centimeter, for example), then a significant amount of heat could be converted to electricity by a single diode-like device and these devices would have utility as refrigerators or in recovering wasted heat energy from a variety of sources. See Efficiency of Refrigeration using Thermotunneling and Thermionic Emission in a Vacuum: Use of Nanometer Scale Design, by Y. Hishinuna, T. H. Geballe, B. Y. Moyzhes, and T. W. Kenny, Applied Physics Letters, Volume 78, No. 17, 23 Apr. 2001; Vacuum Thermionic Refrigeration with a Semiconductor Heterojunction Structure, by Y. Hishinuna, T. H. Geballe, B. Y. Moyzhes, Applied Physics Letters, Volume 81, No. 22, 25 Nov. 2002; and Measurements of Cooling by Room Temperature Thermionic Emission Across a Nanometer Gap, by Y. Hishinuma, T. H. Geballe, B. Y. Moyzhes, and T. W. Kenny, Journal of Applied Physics, Volume 94, No. 7, 1 Oct. 2003. The spacing between the electrodes must be small enough to allow the “hot” electrons (those electrons with energy above the Fermi level) to flow, but not so close as to allow normal conduction (flow of electrons at or below the Fermi level). There is a workable range of separation distance between 2 and 20 nanometers that allows thousands of watts per square centimeter of conversion from electricity to refrigeration. See Efficiency of Refrigeration using Thermotunneling and Thermionic Emission in a Vacuum: Use of Nanometer Scale Design, by Y. Hishinuna, T. H. Geballe, B. Y. Moyzhes, and T. W. Kenny, Applied Physics Letters, Volume 78, No. 17, 23 Apr. 2001. These references also suggest the advantage of a coating or monolayer of an alkali metal, or other material, on the emitting electrode in order to achieve a low work function in the transfer of electrons from one electrode to the other. This coating or monolayer further reduces the operating temperature and increases the efficiency of conversion.
Mahan showed that the theoretical efficiency of a thermionic refrigerator, using electrodes with a work function of 0.7 eV and a cold temperature of 500 K, is higher than 80% of Carnot efficiency. See Thermionic Refrigeration, By G. D. Mahan, Journal of Applied Physics, Volume 76, No. 7, 1 Oct. 1994. By analogy a conversion efficiency of the electron tunneling process is expected to also be a high fraction of Carnot efficiency. Carnot efficiency presents an upper bound on the achievable efficiency of thermal energy conversion.
The maintenance of separation of the electrodes at atomic dimensions over a large area has been the single, most significant challenge in building devices that can remove heat from a conductor. The scanning tunneling microscope, for example, requires a special lab environment that is vibration free, and its operation is limited to an area of a few square nanometers. Even very recently, all measurements of cooling in a working apparatus have been limited to an area of a few square nanometers. See Measurements of Cooling by Room Temperature Thermionic Emission Across a Nanometer Gap, by Y. Hishinuma, T. H. Geballe, B. Y. Moyzhes, and T. W. Kenny, Journal of Applied Physics, Volume 94, No. 7, 1 Oct. 2003.
Hence, there remains a need for a device which cost-effectively and efficiently converts heat energy into electrical energy in a package that is convenient to use for both the heat source as input and the electrical circuits needing power as output. Abundant sources of heat, including waste heat, could easily become sources of electricity. Examples where employing such devices would help the environment, save money, or both, include:
(1) Conversion of the sun's heat and light into electricity more cost effectively than photovoltaic devices currently used. Many articles describe the use of high temperature thermionic emission to recycle thermal energy from solar collectors by using such heat conversion devices. See Thermionic Refrigeration, By G. D. Mahan, Journal of Applied Physics, Volume 76, No. 7, 1 Oct. 1994; and Multilayer Thermionic Refrigerator, By G. D. Mahan, J. A. Sofao and M. Bartkoiwak, Journal of Applied Physics, Volume 83, No. 9, 1 May, 1998. However such conversions could be less costly and more prevalent if tunneling were achieved at naturally occurring temperatures.
(2) Recovery of the heat generated by an internal combustion engine, like that used in automobile, back into useful motion. Some automobiles available today, called hybrid gas-electric automobiles, can use either electrical power or internal combustion to create motion. About 75% of the energy in gasoline is converted to waste heat in today's internal combustion engine. A tunneling conversion device could recover much of that heat energy from the engine of a hybrid automobile and put it into the battery for later use. U.S. Pat. No. 6,651,760 (Cox, et al.) teaches a method of converting the heat from a combustion chamber and storing or converting the energy to motion.
(3) Reducing the need for noxious gases to enter the atmosphere. The more energy-efficient hybrid automobile is a clear example where noxious exhaust gases escaping into the atmosphere can be reduced. A device that converts engine and exhaust heat of the hybrid engine and then stores or produces electricity in the hybrid battery would further increase the efficiency of the hybrid automobile and reduce the need to expel noxious gases. Coolants used in refrigeration are other examples of noxious gases that are necessary to remove heat, and tunneling conversion devices could reduce the need for emission of noxious gases.
(4) Recovery of heat energy at a time when it is available, then storing it as chemical energy in a battery, and then re-using it at a time when it is not available. Tunneling conversion devices could convert the sun's energy to electricity during the day and then store it in a battery. During the night, the stored battery power could be used to produce electricity.
(5) Power generation from geothermal energy. Heat exists in many places on the surface of the earth, and is virtually infinitely abundant deep inside the earth. An efficient tunneling conversion device could tap this supply of energy.
(6) Production of refrigeration by compact, silent and stationary solid state devices, where such a tunneling device could provide cooling for air conditioners or refrigeration to replace the need for bulky pneumatic machinery and compressors.
(7) Power generation from body heat. The human body generates about 100 watts of heat, and this heat can be converted to useful electrical power for handheld products like cell phones, cordless phones, music players, personal digital assistants, and flashlights. A thermal conversion device as presented in this disclosure can generate sufficient power to operate or charge the batteries for these handheld products from heat applied through partial contact with the body.
(8) Electrical power from burning fuel. A wood stove generates tens of thousands of watts of heat. Such a tunneling device could generate one or two kilowatts from that heat which is enough to power a typical home's electric appliances. Similar applications are possible by burning other fuels such as natural gas, coal, and others. Then homes in remote areas may not require connection to the power grid or noisy electrical generators to have modern conveniences.
The challenge in bringing two parallel electrodes together within less than 20.0 nanometer separation gap requires attention to two parameters. One is the surface roughness and the other is the surface flatness. Surface roughness is the deviation from smoothness in a small, local area. Holes and scratches are examples of deviations that affect surface roughness. Surface flatness is the deviation from parallelism over a large area. Warping, bending, creeping are examples of deviations that affect surface flatness.
When two rigid materials are polished flat using the best techniques available today for integrated circuits, the surface flatness is on the order of micrometers over a square centimeter area. Furthermore, heat and other stresses can cause changes in warping and bending over time, presenting a further challenge in maintaining uniform separation once achieved. A polished metal or semiconductor surface using today's techniques can easily achieve a roughness of less than 0.5 nanometers.
The state of the art of a tunneling energy conversion device suffers from one or more of the following limitations: (1) a separation that is too large for tunneling, (2) an area that is too small for significant energy conversion, (3) layers of solid material that cannot be thermally isolated resulting in low conversion efficiency, and (4) a design that is too complex to manufacture cost effectively.
A separation of 10 microns or more has been achieved by many thermionic systems, but these systems only operate at very high temperatures, require a costly design for safety, and are limited to environments where this temperature is achieved.
A separation of about 2.0 to 20.0 nanometers has been achieved by a method taught in U.S. Pat. No. 4,343,993 (Binnig, et al.) in the design of the scanning tunneling microscope, but the effective area was on the order of a few square nanometers. Such area was too small (compared to the desired area of about one square centimeter or more) to allow enough current to flow through, even in the most optimal of materials, to convert significant energy.
The semiconductor industry teaches and employs many methods for controlling physical parameters like film thicknesses that are on the order of several nanometers. Thermoelectric devices are an example of integrated circuits that convert energy with a stack of layered materials. See Design and Characterization of Thin Film Microcoolers, by Chris LaBounty, Ali Shakouri, and John E. Bowers, Journal of Applied Physics, Volume 89, No. 7, 1 Apr. 2001. However, these methods all require solid materials to be in contact with each other in layers. The heat flows easily from layer to layer, limiting the temperature difference and the conversion efficiency. Because the two electrodes are in contact, the design is at the mercy of available thermoelectrically sensitive materials, and the energy barrier for the electrons to traverse cannot be arbitrarily configured, as is possible by setting the width of a vacuum gap. The materials having needed properties are exotic and expensive elements like bismuth and telluride. For these reasons, thermoelectric devices are limited to a high cost per watt of cooling power and a low efficiency of about 7 percent.
The art of separating two conductors by about 2.0 to 20.0 nanometers over a square centimeter area has been advanced by the use of an array of feedback control systems that are very precise over these distances. A control system includes a feedback means for measuring the actual separation, comparing that to the desired separation, and then a moving means for bringing the elements either closer or further away in order to maintain the desired separation. The feedback means can measure the capacitance between the two electrodes, which increases as the separation is reduced. The moving means for these dimensions is, in the state of the art, an actuator that produces motion through piezoelectric, magnetostriction, or electrostriction phenomena. U.S. Pat. No. 6,720,704 (Tavkhelidze, et al.) describes such a design that includes shaping one surface using the other and then using feedback control systems to finalize the parallelism prior to use. Because of the elaborate processes involved in shaping one surface against the other and the use of multiple feedback control systems to maintain parallelism, this design approach is a challenge to manufacture at a low cost.
Other methods have been documented in U.S. Pat. No. 674,003 (Tavkhelidze, et al.), and US Patent Applications 2002/0170172 (Tavkhelidze, et al.), and 2001/0046749 (Tavkhelidze, et al.) that involve the insertion of a “sacrificial layer” between the electrodes during fabrication. The sacrificial layer is then evaporated to produce a gap between the electrodes that is close to the desired spacing of 2 to 20 nanometers. These three methods are either susceptible to post-fabrication fluctuations due to warping or thermal expansion differences between the electrodes, or require the array of actuators to compensate for these fluctuations.
Another method of achieving and maintaining the desired spacing over time is documented in U.S. Pat. No. 6,876,123 (Martinovsky, et al.) and in US Patent Application No. 2004/0050415 through the use of dielectric spacers that hold the spacing of a flexible electrode much like the way poles hold up a tent. One disadvantage of these dielectric spacers is that they conduct heat from one electrode to the other, reducing the efficiency of the conversion process. Another disadvantage of this method is that the flexible metal electrodes can stretch or deform between the spacers over time in the presence of the large electrostatic forces and migrate slowly toward a spacing that permits conduction rather than tunneling or thermionic emission.
There remain continuing and difficult challenges in meeting the requirements for achieving and maintaining electrode spacing at less than 20.0 nanometer separation gaps, and in mass-producing low cost thermo-tunneling devices, in spite of efforts to date.
An additional utility for a device that can move electrons across a vacuum gap (in addition to providing cooling directly) is to place this gap on top of the thermoelectric stack. In this combination, the hot side and the cool side of the thermoelectric gap become thermally insulated and hence more efficient. A device with a combination of thermoelectric materials and a vacuum gap can provide cooling or heat conversion via thermoelectric methods, thermo-tunneling methods, thermionic methods, or a combination of these methods.
A need, therefore, exists for an improved design for maintaining separation between electrodes in tunneling, diode, and other devices that is more efficient and less costly than existing designs. In particular, a need exists for a design having closely spaced electrodes with a uniform gap. More particularly, a need exists for a design having a pair of electrodes which self-position and self-align at a close spacing gap between them to enable the transfer of electrons across the gap by tunneling, thermionic, or other emission, possibly in combination with thermoelectric elements.